Split gears, unfortunately, often occur with plastic gears especially the older an engine gets it seems. They get brittle and crack.
I've provided a link to the Bachmann website-they offer a catalogue of parts and may have the replacement gear you require. You can also call them toll-free (the number is on the site) and provide the nice person with the model of your engine.
A little known fact is that Bachmann used to provide a lifetime guarantee on their locomotives. If the part you require is no longer made, they sometimes offer a replacement engine (new) at a substantial discount. You will have to mail in your old one. I used this service one time and received a brand new engine for about 60% off the cost of it new. A good deal!
My only other suggestion is to check in your area for local model train shows. You can search for them online. Most of the vendors are hobbyists themselves that sell model trains and supplies to make a few dollars on the weekend. Their trains are usually used but sell for a fraction of the price new. Sometimes you can purchase a beat up engine for its parts for a couple dollars, and sometimes they have parts that have been taken off used engines themselves. Bring your engine along with you and see what you can find. I find bringing an old engine back from the dead always very rewarding! Hope this helps! Happy modeling!
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